1. Field
Exemplary embodiments of the present disclosure relate to a thermal fixing device and to an image forming apparatus, such as a copier, a printer, a facsimile machine, or a multifunctional device having at least two of the foregoing capabilities employing the image forming apparatus.
2. Description of the Background Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction apparatuses having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. In such an image forming apparatus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
Recently, there is increased market demand for high-speed, energy-efficient image forming apparatuses. In order to achieve such required performance, it is important to enhance the heating efficiency of the fixing device used in the image forming apparatus. In such an image forming apparatus, the fixing device may include contact heating systems such as a heating roller system, a film heating system, or an electromagnetic induction heating system.
For example, thermal fixing devices employing a heating roller system basically have a pair of rollers: a fixing roller and a pressure roller. The fixing roller has a heat source, e.g., a halogen lamp, therein and is maintained at a predetermined temperature by the heat source. The pressure roller, which provides the rotational force to rotate the fixing member, is pressed against the fixing roller to form the pair of rollers (for example, see JP-2000-131995-A). A recording material bearing an unfixed toner image is introduced into a contact portion, i.e., a nip between the pair of rollers and heat and pressure are applied to fix the toner image on the recording material.
For fixing devices employing a film-heating system, the recording material comes into close contact with a heating member fixedly supported by a support member via a thin, heat-resistant rotatable fixing film. While sliding the fixing film over the heating member, heat from the heating member is transferred to the recording material via the fixing film (for example, see JP-S63-313182-A and JP-H01-263679-A). In the film-heating fixing device, the heating member may be, for example, a ceramic heater including a resistive layer formed on a ceramic substrate made of alumina or aluminum nitride having properties, such as heat resistance, insulating properties, and high thermal conductivity. Since a thin fixing film having low heat capacity can be used in the film-heating fixing device, the fixing unit has a heat transfer efficiency higher than that of the heating-roller fixing device. Such a configuration can shorten warm-up time, thus allowing quick starts and saving energy.
Fixing devices employing an electromagnetic induction-heating system use a technique in which Joule heat is generated by an eddy current generated in a metallic layer (heat generation layer) of a fixing sleeve using magnetic flux (for example, see JP-H08-022206-A, JP-S51-109739-A, and JP-4302465-B). In an electromagnetic induction-heating fixing device, heat is directly generated in the fixing film using an induction current, thus achieving a more efficient fixing process than the heating-roller fixing device having a halogen lamp as the heat source.
Moreover, in the induction-heating fixing device, a high-frequency induction heating unit quickly raises the temperature of ferromagnetic material contained in an adhesive to the Curie point. Upon reaching the Curie point, the ferromagnetic material loses magnetism. As a result, the temperature of the ferromagnetic material does not rise and is maintained at a constant temperature. Since the Curie point of the ferromagnetic material is set to substantially the same temperature as the fixing temperature, the temperature of the ferromagnetic material is maintained at substantially the fixing temperature. Such a configuration can shorten the start-up time of the rotary heating member and achieve high-precision temperature control without reducing, for example, the separation performance and heat resistance of the surface of the rotary heating member, which are required for the fixing device, and without using a complicated control device.
As described above, such an electromagnetic-induction-heating fixing device may include a fixing sleeve having a release layer, an elastic layer, and a metallic layer (heat generation layer), and a fixing roller having an elastic layer and a support member (metal core). The fixing roller is enclosed in the fixing sleeve, and the fixing roller and the pressure roller are pressed against each other via the fixing sleeve to form a contact portion (referred to as a fixing nip). The fixing sleeve is rotatably provided around the outer circumferential surface of a stationary fixing member. As the pressure roller rotates, the fixing sleeve slides over the outer circumferential surface of the stationary fixing member to convey the recording material. In this configuration, the fixing sleeve is heated by induction heating or other heating methods from the inside or outside of the fixing member.
In the fixing member having the above-described configuration, a ring having a diameter larger than that of the fixing sleeve is provided at an end of the fixing roller to prevent the fixing member from shifting in a thrust direction, i.e., axial direction thereof. For the fixing device using the thin fixing sleeve as described above, typically, the heat capacity of the fixing sleeve is relatively small. Accordingly, when the heating operation is continuously performed with the fixing sleeve stopped, a heated portion in a circumferential direction of the fixing sleeve may exceed an allowable temperature limit of the fixing sleeve in a very short time (for example, in several seconds). Therefore, it is necessary to constantly rotate the fixing sleeve when it is heated and to immediately stop heating when the rotation of the fixing sleeve stops or slows down due to a malfunction. Accordingly, it is preferable that the fixing device be able to detect the rotation of the fixing sleeve or the fixing roller.
However, for the thin fixing sleeve, it is difficult to directly detect the rotation of the thin fixing sleeve supported on the elastic layer of the fixing roller or a housing of the fixing member, and implementation of such a direct detection mechanism is costly. Therefore, conventionally, no detection is performed on the rotation of the thin fixing sleeve, or the rotation of the fixing sleeve is detected by alternative methods, such as detecting the driving force applied to the fixing device or detecting the rotation of the shaft of the fixing roller.
However, even if the fixing roller is driven without any problem and the rotation thereof is detected, for example, a failure may occur in the elastic layer or the interface between the elastic layer and the metal core of the fixing roller, causing the fixing sleeve alone to stop or slows down abnormally. In such a case, the heating unit cannot be stopped safely by the above-described conventional methods.